1. Lactose intolerance: Some people may lack lactase, an enzyme that can break down lactose. When the body can't break down lactose, there will be symptoms such as diarrhea and gastrointestinal discomfort. In this case, it is suggested to reduce the intake of milk or choose lactose-free milk.

2. Allergy: Some people may be allergic to some protein in milk, such as whey protein or casein. In this case, it is recommended to avoid drinking milk or choose other animal dairy products.

3. Bacterial contamination: If the milk is not properly preserved or the hygienic standards are not observed in the preparation process, it may lead to bacterial contamination. This can also lead to diarrhea, gastrointestinal discomfort and other symptoms. Therefore, it is recommended to choose fresh and hygienic milk and pay attention to heating and disinfection before drinking.

If the symptoms of diarrhea are relieved or disappeared after drinking milk, it may be because the body gradually adapts to lactose or allergens. However, this does not mean that the problem is completely solved, and attention should still be paid to controlling intake or choosing dairy products without lactose or low allergens. If the symptoms of diarrhea are serious or last for a long time, it is recommended to consult a doctor for more professional advice.